1. Field of the Invention
The present invention relates to a wireless communication system.
2. Description of the Related Art
As a demand for broadband communication service increases, changeover to a new standard communication technique for supporting high speed and large amounts of data is under consideration. When the new standard communication technique is introduced, the new system is subject to the limited coverage area during a transition period until a national network is built. Hence, when using a service, such as Voice over Internet Protocol (VoIP) in the new system, a user who is on the move can suffer from service interruption because of the limited coverage area.
To address this problem, the standard defines Voice Call Continuity (VCC) and Single Radio VCC (SRVCC) between the new network and the established network having the relatively wide coverage area. The established network considered in the VCC technology and the SRVCC technology indicates a Circuit Switched (CS) network, and the new network indicates a Packet Switched (PS) network. The VCC switches a voice call between a CS domain and an IP Multimedia Subsystem (IMS), and provides functions for voice call origination and voice call termination, and a function for the domain switches between the CS domain and the IMS. The SRVCC is the VCC between the PS access based IMS and the CS connection for the call anchored in the IMS when a terminal can transmit and receive over only one access network during a given time.
According to the VCC technology and the SRVCC technology, an IMS server functions as an anchor of every call. For example, not only the VoIP call over the new network but also the CS call are serviced based on the IMS, rather than the existing Public Switched Telephone Network (PSTN). The VCC allows bidirectional switching between the VoIP call and the CS call. However, for the bidirectional switching, the CS call should always use the IMS server without using the PSTN network. Accordingly, the air interface uses the CS network, and the core network uses the PS-based IMS server. Because of the difference of the communication manners, network efficiency can deteriorate and the performance of the voice service can be degraded. In addition, since voice handover is carried out in the IMS stage, not in the air interface, voice data can be lost thereby degrading voice quality by causing temporary voice interruption.
While the SRVCC can selectively use the CS call via the PSTN network and the call via the IMS server with a Mobile Switching Center (MSC), it supports only the unidirectional switching from the VoIP call to the CS call. For example, when the terminal leaves the coverage area of the new network during the call setup within the new network coverage area, the call is switched to the established network. However, the call is not switched to the VoIP call even when the terminal re-enters the new network coverage area. In this case, the CS call is anchored in the IMS server.
Furthermore, a new interface between the MSC, which controls the call in the CS network, and the IMS server, which controls the call in the PS network, needs to be configured. The SRVCC technology, which is based on the handover between heterogeneous networks, further needs interfaces between the two networks for the handover. For example, interfaces, such as 1xCircuit Switched (1xCS) InterWorking Solution function (IWS) and S102, are needed between a Code Division Multiple Access (CDMA)1x network and a Long Term Evolution (LTE) network, and interfaces, such as S3 and S3-CS, are needed between a Wideband CDMA (WCDMA) network and the LTE network.
Another problem of the related art is that, when the PS network and the CS network adopt different vocoders, transcoding in the core network needs to be performed, or one network needs to be modified in order to use the same vocoder in both networks. For example, the LTE network and the CDMA1x network adopt different vocoders of Adaptive Multi Rate (AMR) audio codec and Enhanced Variable Rate Codec (EVRC). The VCC should perform the transcoding until the voice call terminates after the VCC of the IMS server, or the vocoders of the two networks should be identical. The former increases load of the core network, and the latter needs to extend the vocoder supported by the terminal. The core network of a provider basically supports the transcoding in the communication with a subscriber of other provider. However, since the VCC needs the transcoding even in the communication between the subscribers of the same provider, the load of the core network greatly increases. Similar to the VCC, the SRVCC needs the transcoding in the core network or the identical vocoder. Yet, the SRVCC can allow the existing voice call over the PSTN. When the vocoder is standardized to the AMR, the CS network should support both of the AMR and the EVRC.
As discussed above, the call service provision merely with the new system cannot ensure the call continuity during the transition period until the new system establishes the nationwide network. Thus, the VCC or SRVCC technology over the established network needs to be used as aforementioned. However, the current standard VCC technology and SRVCC technology need additional interfaces in the new network and the established network. In addition, the handover can cause connection delay. Furthermore, data can be lost and the identical vocoder is problematic. Hence, what is needed is a solution for addressing the problems, such as additional interface construction, connection delay, data loss, and identical vocoder.
Therefore, a need exists for an apparatus and a method for providing voice call continuity using different networks in a wireless communication system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.